1. Field of the Invention
The present invention relates to a single- or multi-layered tubular, unstretched mono- or biaxially oriented (and heatset) food casing made from a thermoplastic mixture which comprises at least one aliphatic polyamide and/or copolyamide and at least one or more synthetic water-soluble polymers and at least one organic and/or inorganic filler. On account of its high water vapor permeability, it is suitable in particular as artificial sausage casing for raw sausage varieties.
2. Description of Related Art
Food casings, especially sausage casings, are produced from natural skin, textile skin, fibrous skin or cellulose skin, collagen or plastic. Although the collagen or hide fiber skin is distinguished by a natural surface and a pleasant haptic quality, it is produced from cattle hides by a very complex and environmentally-polluting method. The hide tissue is digested to fibrils using acids (e.g. lactic acid); the high-viscosity mass is extruded and, using gaseous ammonia or ammonium hydroxide, slowly and compactly precipitated and solidified. On drying, crosslinking (curing) then takes place in order to give the products a sufficient stability, so that they withstand the scalding process without significant loss in strength. Natural skins, and also hide fiber skins, however, are finding increasingly less acceptance from the end consumers because of various incidents such as BSE disease in cattle and the misuse of antibiotics. In addition, legal restrictions are threatened. An alternative to the mentioned skins is therefore desirable. Cellulose skins, even those with fiber reinforcement, can only take over this task with restrictions. The production process is also no less complex and environmentally harmful than the collagen process.
Food casings based on synthetic polymers can, in contrast, be a real alternative. These can be produced very simply, cheaply and hygienically safely via a combined extrusion and film-forming method. However, pure plastic casings, owing to their unnatural, smooth and glossy surface, have not been able to prevail in the market sector for collagen or natural skin. In addition, they can store only a little water and show only low permeability to water vapor, which is not sufficient for raw sausage applications, since there a certain degree of drying must be achieved. In salami ripening, for example, a weight loss of about 23 to 26% after 10 ripening days is customary, which requires high-permeability casings. Furthermore, conventional plastic casings are not permeable to cold smoke (approximately 20-35° C.) and are permeable to only small amounts of hot smoke (approximately 70-80° C.). Cold smoking, however, is a standard method in the case of raw sausage varieties.
Films made from water-soluble polymers, such as poly(vinyl alcohol) and natural materials, for example starch, are described in great number. U.S. Pat. No. 5,322,866 discloses a biodegradable poly(vinyl alcohol)-starch film which, however, does not exhibit water strength. Enhanced water resistance can be achieved by adding a crosslinker or by coating the film with water-repellent substances (U.S. Pat. No. 5,106,890).
By far less moisture-susceptible are blends of water-resistant synthetic polymers, e.g. polyamides and water-soluble polymers to which, if appropriate, natural materials are also admixed. A crosslinker is generally no longer necessary in these cases, since the water-insoluble matrix substantially shields the moisture-sensitive substances from their environment.
For instance, WO 94/16020 describes biodegradable blends of two polymers of which each is itself already biodegradable. As first polymer, mention is made, inter alia, also of polyamide (nylon), as second component, inter alia, poly(vinyl alcohol) (PVAL), polylactide and other aliphatic polyesters. Optionally, in addition, a polysaccharide can be admixed. On extraction with water, the poly(vinyl alcohol) fraction is extracted from articles produced from said blends so that, a high-porosity sponge-like surface, visible under the electron microscope, results. Such a material is therefore unsuitable for the use for films for coating moist foods.
U.S. Pat. No. 4,611,019 describes blends of thermoplastic poly(vinyl alcohol) and a small fraction of polyamide or polyester which, compared with pure poly(vinyl alcohol) is said to have the advantage of a high gas barrier at low moistures and in particular at elevated moistures.
In the abovementioned prior art, the permeation of (smoke) gases and water vapor does not play a role and is even unwanted. Generally, the biodegradability is in the foreground. Natural appearance and pleasant haptic quality are of secondary importance here.
WO 02/078455 discloses a thermoplastic system made of polyamide and water-soluble polymers in combination with readily smokeable food casings. The use of a natural substance component (as filler or in thermoplasticized form) or of an inorganic filler is not absolutely necessary. Owing to their unnatural, smooth and glossy surface, such food casings have not been able to establish themselves in the market sector for collagen skin or natural skin or fiber skin.